Although the interest in solar energy utilization goes back to earliest history, the first commercial use of solar hot water devices did not find a practical market until early in the 20th century. In the 1950's a revival of solar technologies developed new systems for household heating, water distillation, mechanical pumping, industrial furnaces and new solar photovoltaic cells which brought several new instruments into existance for the direct measurement of solar energy radiant intensity. These instruments were mainly expensive laboratory quality devices for use by solar and biological scientists, the academic research community and meteorological and atmospheric scientists to determine the duration and intensity of solar heating of the earth's atmosphere and surface or the climatology of specific areas. Only limited measurements have so far been made in meterological, climatological, environmental, botanical and biological research studies. The intense national need and dedication within the past decade for the development of alternative energy sources to our fossil-fuel based society has seen the rapid development of economically feasible advanced solar energy systems of many kinds for thermal, electrical and mechanical power production, but little corresponding advancement has occurred in solar energy metering instruments.
A reluctance to rely upon limited data and widely extrapolated estimates of the energy potential of the sun, the failure of some research demonstrations of these systems and a habitual dependence upon a fossil fueled economy, has inhibited the effective public acceptance of this clean and virtually unlimited energy resource. Solar energy systems of many kinds are commercially ready now for widespread cost-effective use, but what is needed to convince the public of its viability and practicality are low cost reliable instruments that can be operated by persons of nominal skills to accurately measure the actual solar energy potential at specific sites of use. No known prior art instrument or systems of this kind now exist which are designed specifically as a complete system for the direct metering and recording of the thermal, electrical and mechanical power potential of the radiant energy from the sun.
In brief review of the technology relevant to this invention, the energy from the sun at the earth's orbital distance has been termed the "solar constant" which averages 1.94 calories per square centimeter per minute, or the equivalent of approximately 77 BTU's per minute or 1.37 kilowatts irradiating each square meter of the earth's hemisphere. The earth's atmosphere attenuates and spectrally filters this energy to less than approximately one-forth this level over the spectra band between 0.2 to 18 micrometers (micron) wavelength. The lower the angle of incidence of the sun's energy with respect to the local surface plane, the greater the atmospheric attenuation. The greatest single modifier of solar radiation is atmospheric moisture and cloud cover which results in diffuse scatter, reflection and direct energy attenuation by conversion to heat so that at the surface the spectral range of nearly all significant solar radiation, about 60 percent, is the shorter wavelength energy in approximately the 0.2 to 3.0 micron range, most of which is directly received on a clear day entirely in the 1/2 degree aperature angle of the sun as observed from the earth, plus approximately 1 degree angle for refraction due to atmospheric turbulence. On cloudy days up to 40 percent of the total available solar energy may be received from diffuse radiation alone within an angle of 30 degrees about the sun location.
Solar Energy Collector Systems may be of many different types but all share a common configuration of having some form of energy absorbing surface or focus the energy on an absorber, either of which may be tilted from the zenith for maximum normal energy incidence and are thoroughly insulated against reradiation, convective or conductive losses of the net absorbed energy. Covers of glass or plastic in multiple or single layers permit the passage of most of the energy in the more intense spectra of 0.2-3.0 microns wavelength and are opaque to or reflect longer wavelengths above 5 microns such as that of reradiation, while insulating against convective losses. Similarly photovoltaic converters have a nonlinier low efficiency response to the shorter wavelength ultraviolet and visible energy spectra up to 1.15 microns, with a peak response at 0.85 microns. The net solar energy thus available for conversion and use in thermal, electric or mechanical systems is basically dependent upon the incident rodient intensity level, times the collector area, times the efficiency of transmittance of covers, less conductive losses, times the absorbtance and emittance and spectral response of the absorbing surface or its efficiency of power conversion, less any losses of energy due to thermal, electrical or mechanical inefficiencies of the collector/converter system. Energy is this power function times the time of production and may be measured in equivalent units of British Thermal units or kilowatt-hours or Horsepower hours.
References may be made to pyranometers, radiometers, or other prior art, solar radiance intensity transducers of many configurations which have remained functionally unchanged over the past several decades but which continued to be technically advanced by an increasing sophistication of modern technology. They have for example become smaller, more accurate and reliable and use new plastic optical materials, thermal-voltaic, thermionic, or photovoltaic sensors, photoelectronic devices, solid-state semiconductors and electronic components to generally improve their basic performance, but at increasing cost. Data processing systems have also been advanced at a remarkable rate so that now a small programable microprocessor or operational amplifier that can be held in the palm of the hand will perform complex data conversion functions automatically and will operate for many hours on a small battery. Similarly, sophisticated recording instruments of many kinds have been developed that are small, mechnical clock motor or battery powered, portable and accurate and can operate in moderate field environments for many hours to produce a permanent record of data of all kinds in digital and analogue formats.
From the need and desire to obtain new measurements and precise data at specific sites or areas for the computation of the available energy or solar system critical design parameters, each of these readily available devices and instruments have been generally contrived into various prior art systems for the metering and recording of the radiant intensity of the sun. Each of these known prior art systems is generally composed of a solar energy transducer and a common recording instrument which has determined the requirements of the data processing elements of the system for the conversion or conditioning of the transducer output signal into convenient measures of hourly average, daily average, total or the instantaneous luminance or radiant intensity values, and then directly into signal levels and kinds required of the specific recorder mechanism. These prior art systems are typically very expensive, technically complex, unreliable, but not entirely self-powered and therefore must have their batteries replaced frequently or are confined in their utility to operation only on commercial or other available electrical power. The recorders now in use must be serviced frequently with replacement components needed to renew their function, and may not function at all in extreme weather conditions. The data collected is often so frequent and extensive that it must be post-processed by lengthly manual planimetrics method or on large scale computers to obtain meaningful information on solar energy levels which adds appreciably to the technical complexity and expense of such measurements. These systems are also a serious maintenance problem due to the need for frequent recalibration of the transducer, poor system reliability and high levels of technical sophistication which require expensive laboratory instruments to calibrate and maintain them. Thus the technical skills generally required to properly install, operate, maintain and utilize the data from these prior art systems are usually those of highly trained engineers and scientists experienced in electronics, data processing, optical pyrometery and solar system sciences. The process of then computing and relating the solar radiance data thus obtained to significant solar system design requirements, performance estimates or the practical consideration of solar energy system alternatives is generally well beyond the capability of most persons of nominal skill.
With the exception of Pyroheliometers and most Radiometers which are configured as sun tracking radiance measuring telescopes, prior art solar radiant intensity metering instruments use transducers which are designed to receive all radiant energy over a full 180 degree hemisphere and serially record total or average energy levels over the entire solar day. They therefore lack directivity or the capability to limit their field of view to only those spherical angles relevant to the direct and narrow diffuse solar radiation required, and when tilted from a horizontal orientation to maximize the incident energy received, are susceptible to specular reflection of energy from the earth and man-made objects in the field of view. Solar Energy Collector Systems are generally not efficient or capable of operating before the sun has reached the usable "solar window" at an angle corresponding to 9 AM solar time or beyond the angle corresponding with 3 PM, and unless specifically designed to utilize reflected energy from a ground-plane reflector to increse the effective aperature are of the collector, such reflected energy from the earth or other objects is spurious and cannot be relied upon as a design source. Consequently these prior art solar radiant intensity metering systems employing such transducers are not efficient in the selection of the kinds and amount of data that they collect and record to only that which is actually needed. Also the practice of averaging the instantaneous radiance data may lead to seriously underestimating the performance of an actual Solar Energy Collector System which would have far greater thermal-inertia or integrating capacity than any of the transducer sensors commonly employed. Therefore, the continuous automatic integration of instantaneous measured values and use of high thermal-inertia sensors is preferred, but not implemented in known prior art systems.
Sensor degredation is a common fault among prior art pyranometer transducers. They are normally composed of a painted surface in Parsons Black with coupled thermopiles, or black and white alternate stripes, or may be a black absorbing surface with contact thermocouples or thermionic voltage generators. These painted surfaces change properties when exposed to continual sunlight and periodically require return to the factory for renewal or replacement. Silicone solar photovoltaic converters also degrade permanently when thus exposed to the sun and atmosphere over long periods of time and therefore require at least annual replacement of the sensor element. Domes and covers on these instruments must be cleaned periodically, and when made of certain plastics are susceptible to ultraviolet induced changes in spectral transmittance or permanent physical damage and should be replaced annually. Therefore, these prior art instruments all require frequent calibration and maintenance as is recommended by the World Meteorological Organization. Otherwise, sensor absorbing surfaces and cover domes should be carefully designed and constructed of such materials that do not degrade when operated in their intended environment. Also, the instruments must be sealed against moisture or use desiccant to prevent evaporation or ice formation within the cover dome which would reduce the measured solar radiant intensity and may result in corrosion of sensor surfaces, thermopiles or bimetalic thermionic generating devices and chemical reactions with painted surfaces. Only the most expensive prior art instruments are so constructed to minimize moisture damage and no known instrument has a non-degrading ideal radiant energy absorbing surface. An ideal absorbing surface would be one which is spectrally broad wavelength sensitive, of a constant performance non-degrading surface whose absorbtance is near 99 percent and emittance, or reradiation of energy, is less than 1.0 percent. Such surfaces exist in research laboratories but these have not been put to practical use in prior art solar radiance measuring instruments.
It is a common prior art practice to utilize the existing solar radiance or insolation data which has been collected almost casually at some universities and meteorological stations widely scattered across the country, and to extrapolate an estimate of the site-specific available solar energy. These measurements have been made infrequently over a short period of time with poorly calibrated instruments and systems and are recorded as average monthly values without regard to the possible end-use of the data. Thus the estimates derived from this data are gross to begin with, and any extrapolation of data between widely separated geographic locations could have enormous errors due to the unique climactic differences in the sites. To overcome some of this deficiency in usable data, elaborate standard data tables, mathematical formulations, analytical simulations and standard methods of estimation have been recently developed by Government and private professional institutions. Even though these methods tend to reduce the total likely error of estimation they are compromised by the limited data format and applicable only to very long-term conservative estimates of average solar collector system performance, which are independent of the climatology, not precise and have typically underestimated the actual usable energy levels that may be obtained by as much as one-half. This has resulted in expensive errors in experimental system design and could lead to inhibiting the effective dvelopment and public acceptance of this important unlimited energy resource. Thus, what is required are new instruments for the accurate measurement of solar energy levels and actual power production potential of collector systems located at specific sites which are versatile, reliable, inexpensive and can be successfully operated by persons of nominal skill to directly obtain the required Solar Energy Collector System performance data.